cli: degrade -getinfo gracefully for older servers

[jonatack]

Follow-up to #19405. Degrade gracefully when -getinfo is called on an older version of bitcoind where getnetworkinfo does not return in/out connections.

before

  "connections": {
    "in": null,
    "out": null,
    "total": 125
  }

after

  "connections": {
    "total": 125
  }

[luke-jr]

This opens a can of worms IMO, and isn't worth it just to hide null values (which should be considered equivalent to omitted ones anyway)

[jonatack]

@luke-jr that could be. I felt I'd be remiss to not at least propose a patch.

[laanwj]

Code review ACK.
No opionion on the concept, I think it's fine to leave these as null when you want to use bitcoin-cli with an older server, but also to hide them.